Thermal stability of carrier centers in various types of transparent conductive ZnO and Ga-doped ZnO films

We investigated the thermal stability of the transparent conductive properties of undoped ZnO and Ga-doped ZnO (GZO) films when they were annealed in a high vacuum with stepwise increasing temperature. The ZnO samples included VO-rich and Zn-rich ZnO films; the primary donors were respectively oxygen vacancies (VO) or Zn atoms highly unsaturated with oxygen atoms. VO-rich ZnO was the most unstable against annealing; resistivity initially within the 10−3 Ω cm range diverged higher than 10 Ω cm when a critical temperature was exceeded. The critical temperature between 350 and 450 °C depended on the film thickness, which indicated that VO's were diminished through recombination with migrating interstitial oxygen atoms. In contrast, Zn-rich ZnO films remained highly conductive up to 550 °C. They became more and more transparent and their crystallinity improved at higher annealing temperatures, which was the consequence of metallic-like Zn atoms being removed through desorption from the surface or being accommodated into the crystalline lattice. Comparatively, GZO films were more robust against annealing with their resistivities remaining unchanged up to 350 °C.